Method for operating basic oxygen steel processes with the introduction of carbon dioxide

ABSTRACT

This invention relates to a method of making steel by the basic oxygen process in which carbon dioxide is injected to form a curtain against the intrusion of atmospheric oxygen and nitrogen and in which high purity carbon monoxide may be recovered from the basic oxygen process off-gases.

United States Patent 11 1 Glassrnan 1 Oct. 30, 1973 METHOD FOR OPERATING BASIC 3,592,630 7 1971 Willett 75/60 OXYGEN STEEL PROCESSES WITH THE 2,819,160 1/1958 Bannister et a1... 75/52 2,958,597 11/1960 Churcher 75/60 X INTRODUCTION OF CARBON DIOXIDE 3,661,559 5/1972 l-lorvath et al..... 75/58 X [75] lnventor: Donald Glassman, Mount Lebanon 3.672.869 6/1972 Niehaus 5/ 0 X Township, Allegheny county, p 3,615,355 10 1971 Skinner 75/60 [73] Ass1gnee: United States Steel Corporation, primary ExaminerL Dewayne Rutledge P1ttsburgh, Pa.

Asszstant Examiner-Peter D. Rosenberg [22] Filed: Oct. 4, 1971 Attorney-Gene Harsh et a1.

[21] Appl. No.: 186,131 1 v [57] v ABSTRACT 52 us. 01. 75/60, 75/59 This invention relates to a method of making steel y 51 1111. c1. ..c21 5/38 the basic Oxygen Process in which carbon dioxide is [58] Field of Search 75/53, 58, 60, 59 injected to form a curtain against the intrusion of mospheric oxygen and nitrogen and in which high pu- [56 Refer n Cited tity carbon monoxide may be recovered from the UNITED STATES PATENTS basic oxygen process off-gases. 3,468,657 9/1969 Philblad 75/59 2 Claims, 1 Drawing Figure 120 63405776 sc/waam //9 /27 f 12s cArALrr/c c0 PRODUCT 0x/04r/0/v UNIT -/24 /20 E E1 s s //3 //6 BOP v E m ans & g a b, CLEAN/N6 3 Lu E l c0 BY-PRODUCT CO2 E g E E a a a I E 3 8 BACKGROUND OF THE INVENTION Large quantities of carbon monoxide are contained in the off-gas from the basic oxygen process (BOP) for making steel. Because of the large concentrations and 7 high volume of carbon monoxide, this gas is desirably recovered in a condition of high purity for use in making valuable chemicals. The off-gases generally contain 70-80 percent of carbon monoxide on a dry basis.

In the prior art, there are available processes for the collection of BOP gas in which the gases are collected in a water-cooled hood which is placed over the mouth of the BOP converter. The lower section of the hood consists of a movable skirt which can be raised and lowered to adjust the clearance between the hood and the converter mouth. During the gas collection period of the oxygen blowing cycle, the hood is adjusted to a minimum practical distance above the converter mouth. This distance is kept as small as possible to minimize the influx of the external gases, however, it must be large enough to prevent fusion of the hood to the converter mouth by splashing metal. Commonly, this gap is maintained in the region of I-2 feet, often at about llfeet. I v

The influx of external gases into'the hood is controlled by adjusting the pressure differential across the gap between the edge of the converter mouth and the hood. This pressure differential is such that some external gases flow into the hood and mix with the off-gases coming from the converter vessel. In some cases, atmospheric air has flowed across the gap and into the hood. In other cases, nitrogen or steam have been introduced around the ring of the lowermost section of the hood curtain toprovide a gas seal against the atmospheric oxygen. These kinds of processes are described in US Pat. No. 3,215,523; other processes and apparatus ass'ociated with the collection of the BOP off-gases are described, in US. Pat. Nos. 3,143,142, 3,177,065, 3,27l,l29, and 3,377,057.

It is especially desirable that the off-gases from the converter not be contaminated with nitrogen. It is considered that the nitrogen contamination in the BOP gases increases the nitrogen content of the steels produced from the BOP process and results in undesirable aging characteristics in such steels. Moreover, it is very difficult to economically remove nitrogen from carbon monoxide. By the practice of my invention, not only is this undesirable contamination with nitrogen avoided, but also a more efficient recovery process for the high purity carbon monoxide is described.

OBJECTS OF THE INVENTION It is among the objects of this invention to provide for the making of steel by the basic oxygen process.

A carbon dioxide atmosphere is maintained to prevent the introduction of nitrogen or other harmful gases into the steel making gas purification areas of treatment. It is another object of this invention to recover from'such a steel making process a high purity stream of carbon monoxide. Another object of this invention is to provide for the improved quality of steel made by such a process by excluding harmful nitrogencontaining gases. A further object of this invention is to provide for the recovery of the carbon monoxide and thecarbon dioxide in the off-gases of the BOP'by simple purification methods. In addition, a particular object of the invention is to provide such a carbon dioxide atmosphere in the section between the basic oxygen converter and the gas collection apparatus'These and other objects apparent from the description of the invention are contemplated by the applicant.

BRIEF DESCRIPTION OF THE INVENTION AN THE DRAWING The invention relates to a process in which primarily carbon dioxide gas is used toprevent the entrance of air or nitrogen or other harmful gases during the making of steel by the BOP and the processing of the offgases produced in the steel making operation. The FIG- URE illustrates in a schematic flow diagram one embodiment of the invention.

THE DETAILED DESCRIPTION In the BOP for making steel, oxygen is blown into the molten bath of steel, iron and flux to react with the carbon therein. This produces off-gases containing carbon monoxide, carbon dioxide, oxygen and water. These gases are emitted from the BOP converter and are then collected so that the carbon monoxide may be recovered. Thepreferred embodiment of this invention is to provide a gas seal of carbon dioxide at the juncturebetween the mouth of the BOP converter and the gas collection equipment. By providing carbon dioxide at this juncture air or nitrogen can be eliminated from the offgas stream. Equally well, however, in other areas of the process, a carbon dioxide blanket can be used to prevent the entrance of nitrogen, e.g. in the event of a leak in the gas collection lines, a mobile repair unit could be used to supply a carbon dioxide atmosphere over such a leak. The carbon dioxide is of low cost, readily available, non-reactive with carbon monoxide or the atmosphere, non-toxic and easily removable from carbon monoxide. It is apparent that it is especially desirable that pure carbon dioxide be used to exclude nitrogen or other harmful gases from the stream of off-gases. However, the presence of carbon monoxide, oxygen or water in small amounts will not materially affect the practice of the invention.

Once the off-gases are collected, they are then further processed by cleaning and compression, after which the gas stream is further separated into its high purity carbon monoxide product and its by-products of carbon dioxide, water, etc. Conventional means can be used to separate the carbon dioxide from the carbon monoxide, however absorption with such. things as potassium carbonate solutions is practical.

The carbon dioxide for the practice of this invention may be obtained as a recycle stream from the processed off-gases; it may also be obtained from an independent source.

With reference to the FIGURE, a BOP converter 111 contains its charge of steel and flux 112 and has a hood 114. Oxygen is admitted to the vessel through lance 113. During the steel making cycle, off-gases are discharged from the converter and flow into the hood. The pressure inside the hood is controlled at slightly less than atmospheric pressure. Carbon dioxide is introduced through the gas distributor in the hood curtain 115. The flow of carbon dioxide is such that a blanket of the gas will fill whatever physical gap existsbetween the hood and the converter. This flow of carbon dioxide will exclude nitrogen or air from the off-gas stream. The particular flow rate of gas will of course depend upon the pressure differential maintained in the hood as well as the physical separation to be filled by the action of the carbon dioxide stream.

Approximately 0.12 standard cubic foot of carbon dioxide per standard cubic foot of carbon monoxide may be drawn into the hood. Somewhat more must be fed to the hood to take care of losses and to provide dilution gas for purging the system. 0.10 0.30 standard cubic foot of carbon dioxide per standard cubic foot of carbon monoxide would ordinarily be sufficient to exclude nitrogen from the off-gas streams. Particularly:

preferred range is in the amounts of 0.18 0.22 standard cubic foot per standard cubic foot of carbon monoxide. Ordinarily, the net requirement of carbon dioxide is more than met by the amount of carbon dioxide produced during subsequent purification, therefore the process will produce as a by-product, carbon dioxide gas. The off-gases are cooled as they flow through line 116 to conventional gas cleaning operations. Here, particulate matter may be removed and the gases may be compressed to a pressure suitable for subsequent processing and the ultimate use of the purified product. This would be in the range of l0 300 psig. After compression the gas flows by line 118 to a catalytic oxidation unit 119 where the small amount of oxygen in the off-gas stream is converted to carbon dioxide by combustion with the carbon monoxide. Gas leaves the catalytic oxidation unit and passes by line 120 to the absorber 121 where the carbon dioxide concentration of the gas stream is reduced by an absorption system. While any conventional absorption systems may be used, hot potassium carbonate systems are practical. These will reduce the carbon monoxide concentrations to 0.05 to 0.2 volume percent. The solution rich with absorbed carbon dioxide flows from the bottom of the absorber by line 122 to the stripper 123 where the solution is regenerated and carbon dioxide is stripped overhead. A portion of the carbon dioxide may be recycled by line 124 to the gas distribution ring and the hood converter. The remainder of the carbon dioxide may be either discharged or used as a chemical raw material.

From the top of the absorber, gases are sent by line 126 to a caustic scrubber 127 for the removal of the last traces of carbon dioxide. Finally, this gas is fed by line 128 to a dryer 129 for water removal. Silica gel or molecular sieve absorbents are particularly useful for water removal. The steps of caustic scrubbing and drying may be eliminated if the carbon monoxide product may contain about 0.05 to 0.2 percent carbon dioxide and be saturated with water, however this depends upon the particular use to which the carbon monoxide product would be used as a feedstock. Of course, the oxygen removal step may also be eliminated if the carbon dioxide user can tolerate about 2 to about 3 percent oxygen. The particular feature of this invention is the sequence of processing the steps to yield the high purity carbon monoxide. By using the carbon dioxide gas to seal out nitrogen, the process permits the offgases to be treated by relatively low cost methods byv first removing the oxygen, and then removing the carbon dioxide and water from the carbon monoxide product stream.

Example A charge of iron and flux isadded to the BOP converter and the process as illustrated in the FIGURE. Oxygen is admitted to the converter and directed toward the surface of the charge through the lance. The off-gas stream from the converter will contain about 778 standard cubic feet of carbon monoxide, 195 standard cubic feet of carbon dioxide and 27 standard cubic feet of oxygen. In the catalytic oxidation unit, the oxygen is converted into carbon dioxide so that about 724 standard cubic feet of carbon monoxide and 249 standard cubic feet of carbon dioxide are fed to the absorber. In the absorber the carbon monoxide is separated from the carbon dioxide. The BOP produces more carbon dioxide than is necessary to provide the sealing between the hood and the converter. Thus, carbon dioxide is available as a by-product. By eliminating the nitrogen from the BOP converter atmosphere, superior aging characteristics in the steels produced may be obtained.

While this invention has been described by specific embodiment and illustrative example, it is not to be deemed limited to same. This invention is more fully set forth in the specification and in the claims and includes such equivalents as would be known to one of ordinary skill in the art.

I claim:

1. In the making of steel by the basic oxygen process and the recovery of the ofi-gases generated therein, the steps comprising: 7 v

a. adding molten iron to an open mouthed converter,

b. moving a collecting hood over the open mouth of said converter,

c. introducing oxygen into said molten iron to convert the iron to steel, while maintaining a curtain of carbon dioxide between said hood and said converter, thereby excluding ambient air from said converter and said hood,

d. collecting the off-gases in said hood, said gases having a composition of at least seventy percent by step t. is conducted by contacting said off-gases with a potassium carbonate solution to absorb said carbon dioxide and then desorbing said solution to provide carbon dioxide. 

2. The process of claim 1 wherein the separation of step f. is conducted by contacting said off-gases with a potassium carbonate solution to absorb said carbon dioxide and then desorbing said solution to provide carbon dioxide. 